Newbie - attempting to encode mp4 clip with no loss of quality, getting black video with audio

Hi, I really have little understanding of this process. But, i have video downloaded that is not recognised in premiere pro. it is vp09, or something. when i am trying to use encoder to give me a usable version with no loss of quality, it creates a black video with perfect audio. i have tried tweaking all sorts of settings to match, but, no cigar. what am i doing wrong? it is a 25fps, 1920x1080p video. not sure if you all need more information? but, i will be studying this whole area further to gain understanding, going forward.